article: In Britain ,people often invite friends for a meal , a party or just coffee. People who know each other very well may visit each other's houses without  an invitation ,but if we invite new friends, usually an invitation is needed. When people invite someone to their homes , they often say ,"Would you like to come for dinner on Saturday ?" Answers are, "Thanks, we'd love to. What time?" or "I'm sorry, We'd love to ,but we have tickets for the concert." However, it is not polite to say,"No, we wouldn't."
  Sometimes, the British use expressions that sound like invitations but which are not invitations. For example. "You must come over for a drink sometime ." or "Let's go out for a meal one of these days."  These are usually just polite ways of ending a talk . They are not real invitations because they don't mention an exact time or day. They just show that the person is trying to be friendly and the answers are ,"Yes , that would be nice ." or "OK, yes ,thanks."
  So next time you hear what sounds like an invitation, listen carefully. Is it a real invitation or is the person just being friendly?
question: It is not polite to answer the real invitation by saying "  _  "
options: Sorry. We'd love to, but we have tickets for the concert . Thanks. We'd love to, what time? Sure. We'd like to. Thanks a lot. No, we wouldn't!